Functional Impact of Nth-like DNA glycosylase on Mitochondrial Dynamics

NTH-like DNA glycosylase (NTHL1), a key enzyme in the base excision repair (BER) pathway, has long been considered essential for maintaining both nuclear and mitochondrial genome integrity. In this study, we integrate in vitro biochemical assays, in-cellulo molecular biology experiments, and bioinformatic analyses to investigate the impact of NTHL1 loss on mitochondrial DNA (mtDNA) stability and function. Our findings challenge the conventional view by showing that the loss of NTHL1 confers a positive phenotype. Although NTHL1 deletion in human cells results in a significant accumulation of mtDNA lesions, it is unexpectedly accompanied by a substantial increase in mtDNA copy number (CN), which correlates with elevated oxidative phosphorylation (OXPHOS) protein levels and enhanced mitochondrial respiratory capacity. Additionally, NTHL1-knockout cells exhibit larger mitochondria and increased levels of the mitochondrial biogenesis regulator peroxisome proliferator-activated receptor {gamma} coactivator 1 (PGC1) and the membrane fusion protein Dynamin-like GTPase OPA1. These changes suggest an adaptive response to stress or altered cellular demands, aimed at enhancing mitochondrial function and capacity. Consequently, NTHL1-knockout cells showed resistance to 1-methyl-4-phenylpyridinium (MPP+)-induced mitochondrial stress along with increased phosphorylation of eIF2. Together, this suggests that the loss of NTHL1 may enhance cellular resilience to oxidative stress by activating protective integrated stress response and mitohormesis pathways thereby contributing to cellular survival under adverse conditions. Collectively, our findings position NTHL1 as a key player in mtDNA stability and mitochondrial function, linking its DNA repair activity to the integrated stress response. Its loss triggers a shift from canonical oxidative phosphorylation to stress-adaptive pathways, ultimately enhancing cellular resistance against oxidative stress through the activation of the mitochondrial hormetic response.